In recent years, a vinyl aromatic monomer-conjugated diene monomer based block copolymer (e.g. SBS: styrene-butadiene-styrene block copolymer, and SIS: styrene-isoprene-styrene block copolymer) is widely used as a solution-type or a hot melt-type base polymer for adhesives or viscous adhesives. For example, in Patent Literature 1 and Patent Literature 2, a viscous adhesive composition and a viscous adhesive composition made from SBS are disclosed. However, a viscous adhesive composition or a viscous adhesive composition made from SBS or SIS has an insufficient balance between workability and adhesion properties such as tack strength, with high melt viscosity, being inadequate from the viewpoints of solubility, heating stability, coating properties, ejection stability, and surface skin quality.
As a method for improving the properties, a viscous adhesive composition containing a triblock copolymer and a diblock copolymer is disclosed in Patent Literature 3 and Non Patent Literature 1.
Further, a viscous adhesive composition including a block copolymer obtained by coupling is disclosed in Patent Literature 4, using a specific bifunctional coupling agent (e.g. aliphatic monoesters and specific dihalogen compounds).
Further, a composition including a block copolymer of a vinyl aromatic hydrocarbon compound and a conjugated diene compound, hydrogenated at a specific ratio, and a tackifier are disclosed in Patent Literature 5.
Further, adhesive compositions including two or more block copolymer compositions are disclosed as an improvement method in Patent Literature 6 and Patent Literature 7.